Identifying a subframe containing information relating to an uplink grant

ABSTRACT

A user equipment (UE) receives, from a wireless access network node of a source cell, a Radio Resource Control (RRC) message identifying a subframe containing information relating to an uplink grant to be sent by a target cell. The UE detects the uplink grant in the identified subframe transmitted by a target wireless access network node of the target cell. The UE sends, in response to the UE successfully accessing the target cell or detecting the information relating to the uplink grant, an RRC message to the target cell to handover from the source cell to the target cell.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 14/269,372, filedMay 5, 2014, which is hereby incorporated by reference in theirentirety.

BACKGROUND

As the demand for wireless data communication using wireless userequipments (UEs) has increased, wireless access service providers areincreasingly facing challenges in meeting capacity demands in regionswhere the density of users is relatively high. To address capacityissues, deployment of small cells has been proposed. A small cell (ormultiple small cells) can operate within a coverage area of a largercell, referred to as a macro cell. A small cell has a coverage area thatis smaller than the coverage area of the macro cell. A small cell (ormultiple small cells) can also operate outside the coverage area of themacro cell or overlap with the coverage area of a large cell.

BRIEF DESCRIPTION OF THE DRAWINGS

Some implementations are described with respect to the followingfigures.

FIG. 1 is a schematic diagram of an example network arrangement thatincludes a cluster of small cells in a macro cell, according to someimplementations.

FIG. 2 is a flow diagram of a process of a user equipment (UE) accordingto some implementations.

FIG. 3 is a schematic diagram of an example subframe according to someimplementations.

FIG. 4 is a flow diagram of a process of a source wireless accessnetwork node according to some implementations.

FIG. 5 is a flow diagram of a process of a target wireless accessnetwork node according to some implementations.

FIG. 6 is a message flow diagram of a process according to furtherimplementations.

FIG. 7 is a flow diagram of a process of a UE according to furtherimplementations.

FIG. 8 is a block diagram of an example arrangement including a UE and awireless access network node according to some implementations.

DETAILED DESCRIPTION

Various types of small cells can be deployed in a mobile communicationsnetwork. For example, a small cell can be a cell to which datacommunications of UEs can be offloaded from a macro cell. As otherexamples, small cells can include pico cells, femto cells, and coverageareas of relay nodes. A pico cell refers to a cell that has a relativelysmall coverage area, such as within a building, a train station, anairport, an aircraft, or other small area. A femto cell is a cell thatis designed for use in a home or small business. A femto cell isassociated with a closed subscriber group (CSG), which specifies thatonly users within the specified group are allowed to access the femtocell. A relay node is used for wirelessly relating data from onewireless entity to another wireless entity.

Generally, small cells employ wireless access network nodes that consumeless power than a wireless access network node of a macro cell. Awireless access network node communicates wireless signaling with userequipments (UEs) within the coverage area of the wireless access networknode. Examples of UEs can include any or some combination of thefollowing: smartphones, personal digital assistants, notebook computers,tablet computers, game appliances, or any other devices capable ofperforming wireless communications

FIG. 1 is a schematic diagram of an example arrangement that includes amacro cell 102 served by a macro wireless access network node 104. Amacro cell has a coverage area provided by the macro wireless accessnetwork node 104. A macro cell has a coverage area that is generallylarger than the coverage area of a small cell provided by a small cellwireless access network node. For example, in FIG. 1, small cells 106and 110 are depicted, where each small cell 106 is served by arespective small cell wireless access network node 108, and each smallcell 110 is served by a respective small cell wireless access networknode 112.

The small cells 106 and 110 differ in that the small cell wirelessaccess network nodes 108 of the small cells are coupled over linksdirectly to core network equipment 114, while the small cell wirelessaccess network nodes 112 of the small cells 110 are connected to anintermediate gateway, such as a home gateway 116, which in turn isconnected to the core network equipment 114. The core network equipment114 can include a control node and one or more data gateways. The datagateway(s) can be coupled to an external packet data network (PDN) 120,such as the Internet, a local area network (LAN), a wide area network(WAN), and so forth.

Links between a wireless access network node and the core networkequipment 114 can be referred to as a backhaul link, which can representa logical communication link between two nodes; the backhaul link caneither be a direct point-to-point link or can be routed through anothercommunication network or node. In some examples, a backhaul link is awired link. In other examples, a backhaul link is a wireless link.

In some examples, the small cells 110 can be femto cells, and the smallcell wireless access network nodes 112 can be femto wireless accessnetwork nodes. In other examples, the small cells 110 can be other typesof cells. The small cells 106 can be pico cells, coverage areas of relaynodes, or small cells to which data communications of the macro cell 102can be offloaded. Although FIG. 1 shows different types of small cells,including the small cells 106 and the femto cells 110, it is noted thatin alternative examples, just one type of small cells can be included inthe macro cell 102.

FIG. 1 also shows a UE 118, which can move around within the macro cell102, and between macro cells. Although just one macro cell 102 is shownin FIG. 1, it is noted that in alternative examples, multiple macrocells 102 are provided. Also, although just one UE 118 is shown in FIG.1, it is noted that in other examples, multiple UEs are present in themacro cell 102.

In the example of FIG. 1, the UE 118 is currently in the coverage areaof one of the small cells 106. A wireless UE 110 within the coveragearea of a small cell (106 or 110) is able to wirelessly communicate withthe respective small cell wireless access network node (108 or 112). TheUE 110 is also able to wirelessly communicate with the macro wirelessaccess network node 104.

When the UE 118 moves, the UE can traverse across multiple small cells,including small cells 106 and 110. When the UE 118 crosses betweendifferent small cells (106, 110), handover can be performed, where theUE 118 is handed over from being served by a serving wireless accessnetwork node in a serving small cell to a target wireless access networknode. A serving wireless access network node of a UE refers to awireless access network node whose associated cell is activelycommunicating with the UE. A target wireless access network node is awireless access network node whose associated cell is determined to be abetter serving cell for further data and control transactions of the UE.Handing over the UE from the source wireless access network node to thetarget wireless access network node involves switching the data and/orcontrol paths of the UE from the serving wireless access network node tothe target wireless access network node when the UE indicates itssuccessful association with the target cell.

If the UE 118 traverses across multiple small cells, then multiplehandovers may be performed. Since the coverage area of each small cellis relatively small, the number of handovers that can occur as a resultof movement of the UE can be relatively high.

Although reference is made to handover between small cells in someexamples, it is noted that techniques or mechanisms according to someimplementations are also applicable to handovers between a small celland a macro cell, or between macro cells.

During a handover, a failure may occur that may prevent a successfulhandover. For example, one such failure is a radio link failure (RLF),which is a failure due to a radio connection between the UE 118 and awireless access network node becoming of such low quality that the radioconnection is lost. Due to RLF, the UE would traditionally have tore-establish a radio connection, which can take a relatively long time,such as hundreds of milliseconds. Radio connection re-establishmentrefers to a procedure to establish a radio connection after a previouslyestablished radio connection has been lost. A radio connection includesvarious resources (e.g. time slots, carrier frequencies, etc.) that canbe used by a UE to communicate data or control information. As a resultof RLF during handover, data communications of the UE 118 can beinterrupted, or a voice call may be dropped.

In accordance with some implementations, techniques or mechanisms areprovided to reduce the interruption time due to an RLF during handoverof a UE between cells, such as small cells. More generally, theinterruption time of communications of a UE can be reduced duringre-establishment of a radio connection re-establishment by usingprocedures or mechanisms according to some implementations. Radioconnection re-establishment can be performed by a UE for variouspurposes, such as in response to RLF during handover, or in response toother causes.

In some implementations, time synchronization can be provided betweenthe various wireless access network nodes, including between the macrowireless access network node 104 and each of the wireless access networknodes 108 and 110.

In accordance with some implementations, to allow for reducedinterruption time when performing radio connection re-establishment,such as due to an RLF during handover, specified information can be sentto a UE such that the UE can use the specified information to morequickly perform radio connection re-establishment. Traditionally, if RLFoccurs during a handover, the UE attempts to synchronize with a selectedwireless access network node (such as the target wireless access networknode to which the UE is to be handed over), and to access the selectedwireless access network node using a random access procedure. Using arandom access procedure to access the selected wireless access networknode is time consuming and can result in a relatively long interruptiontime when performing radio connection re-establishment.

In some examples, the specified information that can be sent to a UE toallow the UE to more quickly perform radio connection re-establishmentincludes information that identifies one or multiple downlink subframescontaining information relating to an uplink grant (or multiple uplinkgrants) that is to be sent by a target cell. The specified informationinforms the UE beforehand (before occurrence of a handover) certainparameters that pertain to a potential target cell (or potential targetcells). Also, the specified information can include a subframe (or arange of subframes) that is used by the target cell for sendinginformation relating to uplink grant(s) to the UE. If radio connectionre-establishment has to occur, the UE can use the specified informationto detect an uplink grant (or uplink grants) in the specified downlinksubframe (or subframes) sent by the target cell.

An uplink grant identifies uplink resources that can be used by the UEto perform communications of data and/or control information. Upondetection of an uplink grant in a downlink subframe specified by thespecified information, the UE can employ resources indicated by theuplink grant to transmit a message to the target cell to perform theradio connection re-establishment without triggering a complete randomaccess procedure. In this way, the radio connection re-establishment,which can be triggered due to RLF during a handover, for example, can beperformed more quickly. Further, by avoiding the random accessprocedure, the generated interference may also be much reduced.

In the ensuing discussion, reference is made to mobile communicationsnetworks that operate according to the Long-Term Evolution (LTE)standards as provided by the Third Generation Partnership Project(3GPP). The LTE standards are also referred to as the Evolved UniversalTerrestrial Radio Access (E-UTRA) standards. Although reference is madeto E-UTRA in the ensuing discussion, it is noted that techniques ormechanisms according to some implementations can be applied to otherwireless access technologies.

In an E-UTRA network, a wireless access network node can be implementedas an enhanced Node B (eNB), which includes functionalities of a basestation and base station controller. Thus, in an E-UTRA network, a macrowireless access network node (e.g. 104 in FIG. 1) is referred to as amacro eNB, while a small cell wireless access network node (e.g. 108 or112) is referred to as a small cell eNB.

In an E-UTRA network, the control node in the core network equipment 114can be implemented as a mobility management entity (MME). An MME is acontrol node for performing various control tasks associated with anE-UTRA network. For example, the MME can perform idle mode UE trackingand paging, bearer activation and deactivation, selection of a servinggateway (discussed further below) when the UE initially attaches to theE-UTRA network, handover of the UE between macro eNBs, authentication ofa user, generation and allocation of a temporary identity to a UE, andso forth. In other examples, the MME can perform other or alternativetasks.

In an E-UTRA network, the data gateway(s) of the core network equipment114 can include a serving gateway (SGW) and a packet data networkgateway (PDN-GW). The SGW routes and forwards traffic data packets of aUE served by the SGW. The SGW can also act as a mobility anchor for theuser plane during handover procedures. The SGW provides connectivitybetween the UE and the PDN 120. The PDN-GW is the entry and egress pointfor data communicated between a UE in the E-UTRA network and a networkelement coupled to the PDN 120. There can be multiple PDNs andcorresponding PDN-GWs. Moreover, there can be multiple MMEs and SGWs.

FIG. 2 is a flow diagram of a process according to some implementations,which can be performed by a UE (e.g. 118 in FIG. 1). The UE receives (at202) a message identifying a subframe (or multiple subframes) containinginformation relating to an uplink grant (or uplink grants) to be sent bya target cell (or more specifically, by a wireless access network nodeof the target cell). Identifying multiple downlink subframes thatcontain information relating to uplink grants can be performed since itis uncertain when a UE will actually arrive at the target cell during ahandover. As a result, the UE is given more opportunities to detect anuplink grant by monitoring any of the identified downlink subframes.Using multiple subframes can improve reliability. Further, providingmultiple subframes may not lead to waste of resources, since when afterthe UE transmits an uplink message to the target cell using the uplinkgrant received in a certain downlink subframe, the target cell will notprovide further grants in other downlink subframes. The serving cell canrequest multiple uplink grants from the target cell in specificsubframes of the target cell. The target cell could override thepre-allocations of multiple subframes if resources are constrained inthe target cell.

Using the information included in the message, the UE detects (at 204)the uplink grant in a downlink subframe (identified in the message) sentby the target cell. Note that the UE can also detect multiple uplinkgrants in respective identified subframes.

The UE then sends (at 206) a message (in resources specified by thedetected uplink grant) to the target cell to complete a handover from asource cell to the target cell, in response to the UE successfullyaccessing the target cell or detecting the information relating to theuplink grant. In some examples, the message sent at 206 can be a RadioResource Control (RRC) Connection Reestablishment message (which canalso be referred to as an RRC Connection Reconfiguration Completemessage). According to E-UTRA, a radio connection established between aUE and an eNB is referred to as an RRC connection. Each of the UE andeNB includes an RRC entity for establishing an RRC connection. RRC isdescribed further in 3GPP TS 36.331, in some examples. Althoughreference is made to an RRC connection as an example of a radioconnection, it is noted that in other examples, other types of radioconnections can be established between a UE and a wireless accessnetwork node.

The RRC Connection Reestablishment message is sent by the UE to thetarget cell to confirm a handover from the source cell to the targetcell. In response to the RRC Connection Reestablishment message, the eNBof the target cell retrieves the UE context (containing variousinformation associated with the UE) and establishes the RRC connectionwith the UE.

An example of a downlink subframe 300 according to some implementationsthat can include an uplink grant is shown in FIG. 3. The subframe 300includes a control region 302 (for carrying control information) and adata region 304 (for carrying data). The downlink subframe 300 can be asubframe identified in the message received at 202 in FIG. 2.

In the example of FIG. 3, the control region 302 of the subframe 300 cancarry a Physical Downlink Control Channel (PDCCH) 306, which carriesdownlink control signaling transmitted by an eNB to a UE. In the exampleof FIG. 3, the PDCCH 306 can include a pointer (or other information)identifying a location in the data region 304 that contains one ormultiple uplink grants. The identified location in the data region 304can include a Physical Downlink Shared Channel (PDSCH) 306, whichcarries downlink data sent by an eNB to a UE.

To allow the UE to find the portion of the control region 302 (or moregenerally, the portion of the subframe 300) that contains informationreferring to an uplink grant (or uplink grants), a special indicator 307can be provided. In the example of FIG. 3, this special indicator 307 ispart of the PDCCH 306. In other examples, the special indicator 307 canbe in another part of the subframe 300.

In some implementations, the special indicator 307 includes a specialRadio Network Temporary Identifier (RNTI), referred to as an RLF-RNTI insome examples. The presence of the special RNTI indicates thatinformation referring to an uplink grant (or uplink grants) is presentin the subframe 300. A UE can search for the special RNTI to find theinformation referring to an uplink grant (or uplink grants).

When the UE determines that radio connection re-establishment is to beperformed, such as due to RLF during a handover, the UE may monitordownlink subframe(s) identified in the message received at 202 (FIG. 2)sent by the target cell, and may search for an RLF-RNTI in such downlinksubframe(s). Once an RLF-RNTI (within a PDCCH) is found, the UE canlisten to a corresponding PDSCH (pointed to by the PDCCH) containing theuplink grant(s). Resources specified by the uplink grant can be used bythe UE to transmit an RRC Connection Reestablishment message.

In the example of FIG. 3, the pointer in the PDCCH may refer to thePDSCH 308 that includes multiple uplink grants (e.g. 310-1, 310-2, . . .310-N in FIG. 3). One or more of the uplink grants 310-1, 310-2, . . .310-N can be for a particular UE. Each uplink grant in the PDSCH 308 ispreceded (or otherwise associated) with a respective UE ID (312-1,312-2, . . . 312-N), which can be a temporary identity of the respectiveUE in a serving cell. In the example of FIG. 3, each UE ID 312-1, 312-2,. . . 312-N can include a cell RNTI (C-RNTI) and a serving cell ID. TheC-RNTI is allocated by an eNB of a serving cell to a UE, and uniquelyidentifies the UE within the serving cell. The serving cell ID is anidentifier of the serving cell (different cells have different servingcell IDs). In another alternative, serving cell ID may not be senttherefore the UE ID would include only the C-RNTI or other identifier ofthe UE.

A UE can identify its UL grant(s) in the PDSCH 308 by looking for itstemporary identity within the PDSCH 308. By being able to listen foruplink grants in specific downlink subframes, a UE can perform radioconnection re-establishment more quickly than if a random accessprocedure has to be performed. In this way, the interruption time due toRLF during a handover can be reduced.

FIG. 4 is a flow diagram performed by a serving eNB, according to someexamples. The serving eNB is the eNB that is currently serving a UE,prior to a handover to a target eNB of a target cell.

The serving eNB receives (at 402), from the UE, a handoff priorityindication. The handoff priority indication is an indication sent by theUE to the source eNB to indicate that the UE is to receive a message(e.g. the message sent at 202 in FIG. 2) containing informationidentifying one or more downlink subframes that contain informationrelating to an uplink grant (or uplink grants) (in other words, the UEis to be provided handover priority). Note that not all UEs will sendthe handoff priority indication, just those UEs that have specificcharacteristics. For example, a UE is provided handover priority ifsignal quality measurements as a function of time indicate that the UEhas a mobility greater than a specified threshold. In other examples, aUE is provided handover priority if the UE is engaged in time-sensitivecommunications (e.g. voice calls) or those UEs that have higher prioritylevels (e.g. those UEs with higher quality-of-service levels). Furtherexample criteria are discussed below. In an alternative, the UE mayindicate the handover priority during its call setup stage. In anotheralternative, the UE may indicate the handover priority together with itsmeasurement report. The UE may also indicate the handover priority inresponse to a request from an eNB.

In response to the handoff priority indication, the serving eNB sends(at 404), to the UE, a message identifying one or more downlinksubframes containing information relating to one or more uplink grantsto be sent by a target cell. This message is the message received at 202in FIG. 2 by the UE.

FIG. 5 is a flow diagram of a process of a target eNB, according to someexamples. The target eNB receives (at 502), from a source eNB, ahandover request for handing over a UE from a source cell to a targetcell. The target eNB detects (at 504) that the UE has not successfullyhanded over to the target cell. In an alternative, the detecting can bebased on an implementation specific timer or counter. In response to thedetecting, the target eNB sends (at 506) an uplink grant (or multipleuplink grants) to the UE in a subframe (or multiple subframes)previously identified to the UE by the source eNB (in the message sentat 404 in FIG. 4, for example).

FIG. 6 is a message flow diagram of an example message flow diagram,which involves a UE, a serving eNB, a target eNB, an MME, and an SGW.Initially, while the UE is served by the serving eNB, uplink anddownlink data packets are communicated (at 600) between the UE and theSGW in the core network equipment 114 (FIG. 1) through the serving eNB.

The UE performs (at 602) a handover priority determination, whichdetermines whether the UE is to be given priority in case of radioconnection re-establishment (e.g. RRC connection re-establishment) is tobe performed due to RLF during a handover.

The UE is triggered to send (at 604) a measurement report according torules that are set by in system information (sent by an eNB to UEs), bya standards specification, and so forth. The measurement report caninclude information indicating a quality of a radio link between a UEand each cell that is detectable by the UE. If the handover prioritydetermination (602) indicates that the UE is to be given priority incase of radio connection re-establishment, the UE may include a handoverpriority indication (such as discussed in connection with FIG. 4) in themeasurement report. In another alternative, the UE can indicate thehandover priority in other uplink RRC messages or in response to arequest from an eNB.

Based on the measurement report, the serving eNB makes (at 606) ahandover decision. If the handover decision indicates that the UE is tobe handed over from the serving (or source) eNB to another eNB, theserving eNB selects an appropriate target cell based on measurementreport received from the UE to initiate a handover. The serving cell canfurther confirm (at 608) the handover priority of the UE, based onwhether or not a handover priority indication is included in themeasurement report by the UE.

The serving eNB next issues (at 610) an X2AP: Handover Request messageto the target eNB. X2AP refers to a control plane protocol between eNBsover an X2 interface that links the eNBs. The Handover Request messagecontains information to prepare the handover at the target eNB to whichthe Handover Request message was sent. The Handover Request message caninclude a request for an uplink grant at the target cell for theincoming UE to send the RRC Connection Reestablishment message.

It is noted that in some examples the serving cell and the target cellare assumed to belong to different eNBs (in other words, an inter-eNBhandover is performed).

If the handover priority confirmation (608) performed at the serving eNBconfirms that the UE is to be provided handover priority, the servingeNB can send (at 612) a message (e.g. an RRC Reconfiguration message)that contains partial information of the target cell to the UE. Thepartial information may include one or more downlink subframe numbers(such as in the form of a Subframe Offset) (identifying respective oneor more downlink subframes) containing an uplink grant (or uplinkgrants) that is (are) to be sent by the target cell. To accuratelyidentify the subframes in the target cell, a subframe number incombination with a subframe index may be used.

The subframe numbers that the serving eNB provides to the UE can bepreconfigured by the serving eNB. The serving eNB may not wait for theresponse to the Handover Request message from the target eNB but ratherassumes the target eNB may transmit the uplink grants (to the UE'sRLF-RNTI) in the corresponding downlink subframes.

Due to dynamic scheduling characteristics and the handover's higherpriority, the target eNB can allocate the uplink grants in thecorresponding subframes for the UE. It should be noted that the UE willonly respond to one of these uplink grants after moving into the targetcell (i.e. after RLF in the source cell). In the case that the resourceis not available in one of these subframes, the target eNB may notprovide the respective uplink grant. In this case, the UE does notreceive any uplink grant from the target eNB (the possibility isrelatively low due to the large resource pool for the uplink and thedynamic scheduling feature), in which case the UE can then follow thenormal RLF recovery procedure and start a random access procedure afterwaiting a predetermined period of time for an uplink grant to the UE'sRLF-RNTI. Since it is unclear when the UE will actually arrive in thetarget cell, multiple subframes may be pre-allocated to transmit theuplink grants. Since the number of UEs to be provided handoff prioritymay be relatively small, consumption of resources associated withsending uplink grants in multiple downlink subframes may not present anissue.

The target eNB performs (at 614) admission control, which can bedependent on the received quality-of-service (QoS) information of aradio access bearer. The admission control can increase the likelihoodof a successful handover, if the resources can be granted by target eNB.The target eNB configures the resources according to the received QoSinformation and reserves a C-RNTI and optionally a random access channel(RACH) preamble.

The target eNB prepares an uplink grant for the incoming UE to send theRRC Connection Reestablishment message. After a certain duration (whichcan be implementation specific or can depend on an RLF timer, a handoverdelay timer, etc.) after receipt of the Handover Request message, if theUE does not successfully handover to the target cell, the target eNBstarts to allocate the uplink resources corresponding to the subframesindicated by the serving eNB and transmits uplink grants in thesesubframes. If the UE successfully completes the handover, the target eNBwould not have to perform the allocation of resources.

The target eNB prepares the handover and sends (at 616) an X2AP:Handover Request Acknowledge message to the serving eNB. The HandoverRequest Acknowledge message includes a transparent container to be sentto the UE as an RRC message to perform the handover. The container caninclude one or more of the following: a new C-RNTI, target eNB securityalgorithm identifiers for the selected security algorithms, a dedicatedRACH preamble, and other information elements (e.g. access parameters,System Information Blocks, etc.). The Handover Request Acknowledgemessage may also include information for forwarding tunnels, in someexamples. The Handover Request Acknowledge message can also identify adownlink subframe(s) in which an UL grant(s) is (are) expected at thetarget cell for the incoming UE to send the RRC ConnectionReestablishment message.

The target eNB also generates the RRC message to perform the handover,e.g. the RRC Connection Reconfiguration message to be sent (at 618) bythe serving eNB to the UE. The RRC Connection Reconfiguration messagecontains mobility control information. In some examples, the mobilitycontrol information can identify the downlink subframe(s) to be sent bythe target cell to carry an uplink grant(s). In other examples, sinceinformation of the downlink subframe(s) has already been sent in themessage at 612, then the information can be excluded from theinformation sent at 618.

The serving eNB attempts to transmit (at 618) the RRC Reconfigurationmessage to the UE. However, due to poor radio conditions, the UE may notreceive the RRC Reconfiguration message. This causes an RLF.

Before handover completes, the serving eNB can continue to exchange (at619) uplink and downlink data packets of the UE with the SGW in the corenetwork equipment 114 (FIG. 1).

The serving eNB next starts transmitting (at 620) unacknowledged datapackets to the target eNB; these data packets are to be sent by thetarget eNB to the UE.

After determining that an RLF has occurred, the UE may move to thetarget cell specified in the partial information included in the RRCReconfiguration message sent by the serving eNB to the UE (at 612).Also, using the partial information, the UE can search (at 622) forPDCCH including information relating to an uplink grant, by searchingfor the RLF-RNTI in an identified downlink subframe (or multiplesubframes) sent by the target eNB. The UE can derive target eNB specifickeys and configure the selected security algorithms to be used in thetarget cell.

When the UE has successfully accessed the target cell or a pointer tothe uplink grant is detected (based on finding an RLF-RNTI in anidentified subframe), the UE sends (at 624) the RRC ConnectionReestablishment message (containing the C-RNTI of the UE) to confirm thehandover with the target eNB. The UE can also send an uplink BufferStatus Report, whenever possible, to the target eNB to indicate that thehandover procedure is completed for the UE. The target eNB verifies theC-RNTI sent in the RRC Connection Reestablishment message.

The target eNB retrieves (at 626) the UE context of the UE, andestablishes an RRC connection with the UE. The target eNB sends (at 628)an S1AP: Path Switch message to the MME in the core network equipment114 (FIG. 1) to inform the MME that the UE has changed cells. S1APprovides a signaling service between an eNB and the MME.

The MME then sends an Update User Plane Request message to the SGW inthe core network equipment 114. The SGW switches the downlink data pathto the target cell. The SGW sends one or more “end marker” packets onthe old path to the serving eNB and then can release any user planeresources towards the serving eNB. The SGW sends an Update User PlaneResponse message to the MME. The MME confirms the Path Switch messagewith a Path Switch Acknowledge message. The target eNB starts making thescheduling decisions on the new packets received from this point.

The target eNB then sends (at 630) an X2AP: UE Context Release messageto the serving eNB to inform the serving eNB of successful handover. Thetarget eNB sends this message after the S1AP: Path Switch Acknowledgemessage is received from the MME.

FIG. 6 shows an example of the uplink data interruption time (634) andthe downlink data interruption time (636), which is reduced due to useof techniques or mechanisms according to some implementations.

FIG. 7 is a flow diagram of a process performed by a UE, according tosome implementations. The UE measures (at 702) a quality of receivedsignals from the serving cell. In response to an event triggeringneighbor cell measurements, the UE measures (at 704) the quality ofreceived signals from the serving cell and potential target cell(s).

The UE then determines (at 706) whether the UE is to have handoverpriority, such as based on the measurements at the serving cell. Thisdetermination can be implementation specific. For example, when the UEis moving from a pico cell to a macro cell, there may be high likelihoodof RLF, so that the UE should set the handover priority. In anotherexample, if the UE observes the signal strength of the serving celldeteriorates quickly or the UE observes its moving speed is high, the UEmay set the handover priority. The triggering conditions can also bedefined in the standards. This could involve one or more criteria suchas signal strength, signal quality, RSRP, RSRQ, Ec/No, Ec/Io, blockerror rates, frame errors, and so forth. In an alternative, the triggermay be based on a combination of criteria, such as some combination ofthe foregoing factors satisfying some associated threshold.

The criteria to consider and respective thresholds can be networkconfigurable, such as specified in a downlink message.

If the UE is to have handover priority, then the UE includes (at 708)the handover priority indication in a measurement report sent to theserving cell. In another alternative, the UE may indicate the handoverpriority in the call setup stage to the serving cell. If the UE is notto have handover priority, then the UE does not include the handoverpriority indication in a measurement report sent to the serving cell.

In response to an event triggering sending of a measurement report, theUE sends (at 710) the measurement report to the serving cell, where themeasurement report includes information relating to the measurements ofreceived signals from the serving cell and the neighboring cell(s).

If an RLF occurs at point 712 in FIG. 7, then the conventional RRCconnection re-establishment procedure is followed.

The UE receives (at 714) an RRC Connection Reconfiguration message (e.g.sent at 612 in FIG. 6) which contains partial information, includingidentifier(s) of downlink subframe(s) containing information relating touplink grant(s) to be sent by a target cell. The UE may reduce (at 716)an RLF timer to reduce the RLF recovery time.

The UE also measures (at 718) a downlink time difference of arrival(TDOA) between the serving cell and the potential target cell(s). Basedupon the measured information, the UE can have a better estimation ofthe uplink transmission timing for the target cell. In a small cellscenario, since the small cells and the macro cell are likelysynchronized, and the propagation delay is not significant in the smallcells, a fairly accurate uplink timing estimation can be achieved usingthe DL TDOA measurements. The UE may deduce that the target cell issynchronized with respect to the serving cell based on a number ofmechanisms. For example, synchronization may be assumed:

-   -   (1) the UE receives an RRC Connection Reconfiguration message        with partial information (such as the message sent at 612 in        FIG. 6), or    -   (2) if Almost Blank Subframes are configured in the system (such        as described according to 3GPP TS 36.311).

If the target cell is not synchronized with the serving cell, then theserving cell may ignore the handover priority request from the UE andproceed to perform conventional RLF recovery.

If an RLF occurs at point 720, then if standards define a recommendeddownlink subframe(s) in which a target cell is to transmit uplinkgrant(s), then the UE can listen (at 722) to such recommended downlinksubframe(s) to detect uplink grant(s) from the target eNB. Task 722 isomitted if the standards do not define such recommended downlinksubframe(s).

The UE also determines (at 724) whether the UE received a message (e.g.612 in FIG. 6) that identifies subframe(s) that contain informationrelating to uplink grant(s). If not, then the conventional RRCconnection re-establishment procedure is performed (at 726).

The UE determines (at 728) whether a PDCCH referring to uplink grantinformation is successfully decoded (such as due to finding anRLF-RNTI). If not, then the conventional RRC connection re-establishmentprocedure is performed (at 726). However, if the PDCCH is successfullydecoded, then the UE can use the corresponding uplink grant to completethe handover to the target cell.

FIG. 8 is a block diagram of an example arrangement including the UE 118and an eNB 802 (which can be a macro eNB, e.g. 104 in FIG. 1, or a smallcell eNB, e.g. 108 or 112 in FIG. 1).

The UE 118 includes one or more processors 804. A processor can includea microprocessor, microcontroller, processor module or subsystem,programmable integrated circuit, programmable gate array, or anothercontrol or computing device. The processor(s) 804 can be coupled to acommunication component 806 to allow the UE 118 to communicate over anetwork. The processor(s) 804 can be coupled to a non-transitorymachine-readable storage medium (or storage media) 808, which can storemachine-executable instructions 810 that are executable on theprocessor(s) 804.

The eNB 802 includes one or more processors 812, which can be coupled toa communication component 814 and a non-transitory machine-readablestorage medium (or storage media) 816 that can store machine-executableinstructions 818.

The machine-readable instructions 810 and 818 are executable to performvarious tasks as discussed above.

The storage media 808 and 816 can include different forms of memoryincluding semiconductor memory devices such as dynamic or static randomaccess memories (DRAMs or SRAMs), erasable and programmable read-onlymemories (EPROMs), electrically erasable and programmable read-onlymemories (EEPROMs) and flash memories; magnetic disks such as fixed,floppy and removable disks; other magnetic media including tape; opticalmedia such as compact disks (CDs) or digital video disks (DVDs); orother types of storage devices. Note that the instructions discussedabove can be provided on one computer-readable or machine-readablestorage medium, or alternatively, can be provided on multiplecomputer-readable or machine-readable storage media distributed in alarge system having possibly plural nodes. Such computer-readable ormachine-readable storage medium or media is (are) considered to be partof an article (or article of manufacture). An article or article ofmanufacture can refer to any manufactured single component or multiplecomponents. The storage medium or media can be located either in themachine running the machine-readable instructions, or located at aremote site from which machine-readable instructions can be downloadedover a network for execution.

The following provides examples of certain changes that can be made to3GPP standards to provide techniques or mechanisms according to someimplementations. For example, RNTI values in modified Table 7.1-1(underlined text indicates modifications from current standards) of 3GPPTS 36.221 are provided below.

TABLE 7.1-1 RNTI values. Value (hexa- decimal) RNTI 0000 N/A 0001-003CRA-RNTI, C-RNTI, Semi-Persistent Scheduling C-RNTI, Temporary C-RNTI,TPC-PUCCH-RNTI and TPC- PUSCH-RNTI (see note) 003D-FFF3 C-RNTI,Semi-Persistent Scheduling C-RNTI, Temporary C-RNTI, TPC-PUCCH-RNTI andTPC-PUSCH-RNTI FFF4-FFFB Reserved for future use FFFC RLF-RNTI FFFDM-RNTI FFFE P-RNTI FFFF SI-RNTI

Note that the RLF-RNTI can be carried in a downlink shared channel(DL-SCH).

The measurement report sent by a UE as discussed above can be definedaccording to 3GPP TS 36.331 (as MeasurementReport), which can bemodified as indicated by underlined text below.

MeasurementReport Message

MeasurementReport descriptions HandOver-Priority

“1” means the handover of this UE shall be handled with priority. “0”means normal handover applies.

Also, the RRC Connection Reconfiguration message as discussed above andas defined by 3GPP TS 36.331 can be modified as follows (indicated byunderlined text).

The RRC Connection Reconfiguration message is the command to modify anRRC connection. The information element partialMobility ControlInfo isan example of the partial information included in the message sent at612 in FIG. 6.

PartiatlMobilityControlInfo Descriptions SubframeOffset

“n1” means the uplink grant in the target cell is allocated in thesubframe 1. “n2” means the uplink grant in the target cell is allocatedin the subframe 2.The following are the further example changes to various sections of TS36.331 (as indicated by underlined text).

5.3.11.3 Detection of Radio Link Failure

The UE shall:

-   -   1> upon T310 expiry; or    -   1> upon random access problem indication from MAC while neither        T300, T301, T304 nor T311 is running; or    -   1> upon indication from RLC that the maximum number of        retransmissions has been reached:        -   2> consider radio link failure to be detected;        -   2> store the following radio link failure information in the            VarRLF-Report by setting its fields as follows:            -   3> clear the information included in VarRLF-Report, if                any;            -   3> set the plmn-Identity to the RPLMN;            -   3> set the measResultLastServCell to include the RSRP                and RSRQ, if available, of the PCell based on                measurements collected up to the moment the UE detected                radio link failure;            -   3> set the measResultNeighCells to include the best                measured cells, other than the PCell, ordered such that                the best cell is listed first, and based on measurements                collected up to the moment the UE detected radio link                failure, and set its fields as follows;                -   4> if the UE was configured to perform measurements                    for one or more EUTRA frequencies, include the                    measResultListEUTRA;                -   4> if the UE was configured to perform measurement                    reporting for one or more neighbouring UTRA                    frequencies, include the measResultListUTRA;                -   4> if the UE was configured to perform measurement                    reporting for one or more neighbouring GERAN                    frequencies, include the measResultListGERAN;                -   4> if the UE was configured to perform measurement                    reporting for one or more neighbouring CDMA2000                    frequencies, include the measResultsCDMA2000;        -   NOTE: The measured quantities are filtered by the L3 filter            as configured in the mobility measurement configuration. The            measurements are based on the time domain measurement            resource restriction, if configured. Blacklisted cells are            not required to be reported.            -   3> if detailed location information is available, set                the content of the locationInfo as follows:                -   4> include the locationCoordinates;                -   4> include the horizontal Velocity, if available;            -   3> set the failedPCellId to the global cell identity, if                available, and otherwise to the physical cell identity                and carrier frequency of the PCell where radio link                failure is detected;            -   3> if an RRCConnectionReconfiguration message including                the mobilityControlInfo concerning an intra E-UTRA                handover was received before the connection failure:                -   4> include previousPCellId and set it to the global                    cell identity of the PCell where the last                    RRCConnectionReconfiguration including the                    mobilityControlInfo message was received;                -   4> set the timeConnFailure to the elapsed time since                    reception of the last RRCConnectionReconfiguration                    message including the mobilityControlInfo;            -   3> if an RRCConnectionReconfiguration message including                the partialMobilityControlInfo concerning an intra                E-UTRA handover was received before the connection                failure:                -   4> include previousPCellId and set it to the global                    cell identity of the PCell where the last                    RRCConnectionReconfiguration including the                    mobilityControlInfo message was received:                -   4> set the timeConnFailure to the elapsed time since                    reception of the last RRCConnectionReconfiguration                    message including the mobilityControlInfo;                -   4> set the subframeOffset to the subframeOffset                    received in the partialMobilityControlInfo.            -   3> set the connectionFailureType to rlf,        -   2> if AS security has not been activated:            -   3> perform the actions upon leaving RRC CONNECTED as                specified in 5.3.12, with release cause ‘other’,        -   2> else:            -   3> initiate the connection re-establishment procedure as                specified in 5.3.7;

The UE may discard the radio link failure information, i.e. release theUE variable VarRLF-Report 48, hours after the radio link failure isdetected, upon power off or upon detach.

The following describes changes to the RRC connection re-establishmentprocedure as provided by 3GPP TS 36.331 (indicated by underlined text).

5.3.7.2 Initiation

The UE shall only initiate the procedure when AS security has beenactivated. The UE initiates the procedure when one of the followingconditions is met:

-   -   1> upon detecting radio link failure, in accordance with 5.3.11;        or    -   1> upon handover failure, in accordance with 5.3.5.6; or    -   1> upon mobility from E-UTRA failure, in accordance with        5.4.3.5; or    -   1> upon integrity check failure indication from lower layers; or    -   1> upon an RRC connection reconfiguration failure, in accordance        with 5.3.5.5;

Upon initiation of the procedure, the UE shall:

-   -   1> stop timer T310, if running;    -   1> start timer T311;    -   1> suspend all RBs except SRB0;    -   1> reset MAC;    -   1> release the SCell(s), if configured, in accordance with        5.3.10.3a;    -   1> apply the default physical channel configuration as specified        in 9.2.4;    -   1> apply the default semi-persistent scheduling configuration as        specified in 9.2.3;    -   1> apply the default MAC main configuration as specified in        9.2.2;    -   1> release reportProximityConfig and clear any associated        proximity status reporting timer;    -   1> release measSubframePatternPCell, if configured;    -   1> if connected as an RN and configured with an RN subframe        configuration:        -   2> release the RN subframe configuration;    -   1> perform cell selection in accordance with the cell selection        process as specified in TS 36.304 [4];    -   2> if an RRCConnectionReconfiguration message including the        partialMobilityControlInfo concerning an intra E-UTRA handover        was received before the connection failure:        -   2> if the carrierFreq is included:            -   3> consider the target PCell to be one on the frequency                indicated by the carrierFreq with a physical cell                identity indicated by the targetPhysCellId,        -   2> else:            -   3> consider the target PCell to be one on the frequency                of the source PCell with a physical cell identity                indicated by the targetPhysCellId;        -   2> Apply the radioResourceConfigCommon received in the            partialMobilityControlInfo.        -   2> reset TAT timer;        -   Note: in this case, the UE assumes UL synchronization with            target PCell.        -   2> start synchronising to the DL of the target PCell.        -   Note: After DL synchronization, the UE searches for UL            grants in the target PCell in the subframe(s) indicated by            subframeOffset.

5.3.7.4 Actions Related to Transmission ofRRCConnectionReestablishmentRequest Message

If the procedure was initiated due to radio link failure or handoverfailure, the UE shall:

-   -   1> if an RRCConnectionReconfiguration message including the        partialMobilityControlInfo concerning an intra E-UTRA handover        was received before the connection failure        -   2> set the reestablishmentCellId to the tar getPhysCellId in            the partialMobilityControlInfo.    -   1> else        -   2> set the reestablishmentCellId in the VarRLF-Report to the            global cell identity of the selected cell;

The UE shall set the contents of RRCConnectionReestablishmentRequestmessage as follows:

-   -   1> set the ue-Identity as follows:        -   2> set the c-RNTI to the C-RNTI used in the source PCell            (handover and mobility from E-UTRA failure) or used in the            PCell in which the trigger for the re-establishment occurred            (other cases);        -   2> if an RRCConnectionReconfiguration message including the            partialMobilityControlInfo concerning an intra E-UTRA            handover was received before the connection failure            -   3> set the physCellId to the targetPhysCellId in the                partialMobilityControlInfo;        -   2> else            -   3> set the physCellId to the physical cell identity of                the source PCell (handover and mobility from E-UTRA                failure) or of the PCell in which the trigger for the                re-establishment occurred (other cases);        -   2> set the shortMAC-I to the 16 least significant bits of            the MAC-I calculated:            -   3> over the ASN.1 encoded as per section 8 (i.e., a                multiple of 8 bits) VarShortMAC-Input;            -   3> with the K_(RRCint) key and integrity protection                algorithm that was used in the source PCell (handover                and mobility from E-UTRA failure) or of the PCell in                which the trigger for the re-establishment occurred                (other cases); and            -   3> with all input bits for COUNT, BEARER and DIRECTION                set to binary ones;    -   1> set the reestablishmentCause as follows:        -   2> if the re-establishment procedure was initiated due to            reconfiguration failure as specified in 5.3.5.5 (the UE is            unable to comply with the reconfiguration):            -   3> set the reestablishmentCause to the value                reconfigurationFailure;        -   2> else if the re-establishment procedure was initiated due            to handover failure as specified in 5.3.5.6 (intra-LTE            handover failure) or 5.4.3.5 (inter-RAT mobility from EUTRA            failure):            -   3> set the reestablishmentCause to the value                handoverFailure;        -   2> else:            -   3> set the reestablishmentCause to the value                otherFailure;

The UE shall submit the RRCConnectionReestablishmentRequest message tolower layers for transmission.

The following are the further example changes to TS 36.321 (as indicatedby underlined text).

5.3 DL-SCH Data Transfer 5.3.1 DL Assignment Reception

Downlink assignments transmitted on the PDCCH indicate if there is atransmission on a DL-SCH for a particular UE and provide the relevantHARQ information.When the UE has a C-RNTI, Semi-Persistent Scheduling C-RNTI, RLF-RNTI,or Temporary C-RNTI, the UE shall for each TTI during which it monitorsPDCCH and for each Serving Cell:

-   -   if a downlink assignment for this TTI and this Serving Cell has        been received on the PDCCH for the UE's C-RNTI, or Temporary        C-RNTI:        -   if this is the first downlink assignment for this Temporary            C-RNTI:            -   consider the NDI to have been toggled.        -   if a downlink grant for this TTI has been received on the            PDCCH for the RLF-RNTI,        -   if the downlink assignment is for UE's C-RNTI and if the            previous downlink assignment indicated to the HARQ entity of            the same HARQ process was either a downlink assignment            received for the UE's Semi-Persistent Scheduling C-RNTI or a            configured downlink assignment:            -   consider the NDI to have been toggled regardless of the                value of the NDI.        -   indicate the presence of a downlink assignment and deliver            the associated HARQ information to the HARQ entity for this            TTI.    -   else, if this Serving Cell is the PCell and a downlink        assignment for this TTI has been received for the PCell on the        PDCCH of the PCell for the UE's Semi-Persistent Scheduling        C-RNTI:        -   if the NDI in the received HARQ information is 1:            -   consider the NDI not to have been toggled;            -   indicate the presence of a downlink assignment and                deliver the associated HARQ information to the HARQ                entity for this TTI.        -   else, if the NDI in the received HARQ information is 0:            -   if PDCCH contents indicate SPS release:                -   clear the configured downlink assignment (if any);                -   if timeAlignmentTimer is running:                -    indicate a positive acknowledgement for the                    downlink SPS release to the physical layer.            -   else:                -   store the downlink assignment and the associated                    HARQ information as configured downlink assignment;                -   initialise (if not active) or re-initialise (if                    already active) the configured downlink assignment                    to start in this TTI and to recur according to rules                    in subclause 5.10.1;                -   set the HARQ Process ID to the HARQ Process ID                    associated with this TTI;                -   consider the NDI bit to have been toggled;                -   indicate the presence of a configured downlink                    assignment and deliver the stored HARQ information                    to the HARQ entity for this TTI.    -   else, if this Serving Cell is the PCell and a downlink        assignment for this TTI has been configured for the PCell and        there is no measurement gap in this TTI; and    -   if this TTI is not an MBSFN subframe of the PCell or the UE is        configured with transmission mode tm9 on the PCell:        -   instruct the physical layer to receive, in this TTI,            transport block on the DL-SCH according to the configured            downlink assignment and to deliver it to the HARQ entity;        -   set the HARQ Process ID to the HARQ Process ID associated            with this TTI;        -   consider the NDI bit to have been toggled;    -   indicate the presence of a configured downlink assignment and        deliver the stored HARQ information to the HARQ entity for this        TTI.

In the foregoing description, numerous details are set forth to providean understanding of the subject disclosed herein. However,implementations may be practiced without some of these details. Otherimplementations may include modifications and variations from thedetails discussed above. It is intended that the appended claims coversuch modifications and variations.

What is claimed is:
 1. A method comprising: receiving, by a userequipment (UE) from a wireless access network node of a source cell, aRadio Resource Control (RRC) message identifying a subframe containinginformation relating to an uplink grant to be sent by a target cell;detecting, by the UE, the uplink grant in the identified subframetransmitted by a target wireless access network node of the target cell;and sending, by the UE in response to the UE successfully accessing thetarget cell or detecting the information relating to the uplink grant,an RRC message to the target cell to handover from the source cell tothe target cell.
 2. The method of claim 1, wherein the RRC messagereceived by the UE is an RRC Reconfiguration message.
 3. The method ofclaim 2, wherein the RRC message sent to the target cell is an RRCReestablishment message.
 4. The method of claim 1, wherein the subframeincludes a control region and a data region, and wherein the uplinkgrant is in the data region of the subframe.
 5. The method of claim 4,wherein the data region includes plural uplink grants for at least theUE.
 6. The method of claim 5, wherein temporary identifiers areassociated with each of the plural uplink grants, wherein each of thetemporary identifiers includes a cell-RNTI (C-RNTI) of a respective UE.7. The method of claim 1, wherein a temporary identifier is associatedwith the uplink grant, wherein the temporary identifier includes acell-RNTI (C-RNTI) of the UE.
 8. The method of claim 1, wherein the RRCmessage received by the UE identifies plural subframes, the methodfurther comprising: detecting, by the UE, uplink grants in theidentified subframes transmitted by the target wireless access networknode of the target cell.
 9. The method of claim 1, further comprisingsending, by the UE to the wireless access network node of the sourcecell, a handoff priority indication, wherein the RRC message received bythe UE is responsive to receipt of the handoff priority indication bythe wireless access network node of the source cell.
 10. The method ofclaim 9, wherein sending the handoff priority indication is responsiveto the UE determining that one or more criteria are satisfied.
 11. Themethod of claim 10, wherein the one or more criteria comprise a speed ofthe UE exceeding a threshold.
 12. A user equipment (UE) comprising: acommunication interface to receive from a source wireless access networknode of a source cell, a Radio Resource Control (RRC) messageidentifying a subframe containing information relating to an uplinkgrant to be sent by a target cell; and at least one processor configuredto: detect the uplink grant in the identified subframe transmitted by atarget wireless access network node of the target cell; and causesending, in response to the UE successfully accessing the target cell ordetecting the information relating to the uplink grant, an RRC messageto the target cell to handover from the source cell to the target cell.13. The UE of claim 12, wherein the RRC message received from the sourcewireless access network node is an RRC Reconfiguration message, and theRRC message sent to the target cell is an RRC Reestablishment message.14. The UE of claim 12, wherein the subframe includes a control regionand a data region, and wherein the uplink grant is in the data region ofthe subframe.
 15. The UE of claim 12, wherein a temporary identifier isassociated with the uplink grant, wherein the temporary identifierincludes a cell-RNTI (C-RNTI) of the UE.
 16. The UE of claim 12, whereinthe RRC message received by the UE identifies plural subframes, and theat least one processor is configured to: detect uplink grants in theidentified subframes transmitted by the target wireless access networknode of the target cell.
 17. The UE of claim 12, the at least oneprocessor is configured to further sending, to the source wirelessaccess network node, a handoff priority indication, wherein the RRCmessage received from the source wireless access network node isresponsive to receipt of the handoff priority indication by the sourcewireless access network node.
 18. The UE of claim 17, wherein thehandoff priority indication is included in a measurement report from theUE.
 19. A non-transitory machine-readable storage medium storinginstructions that upon execution cause a user equipment (UE) to: receivefrom a source wireless access network node of a source cell, a RadioResource Control (RRC) message identifying a subframe containinginformation relating to an uplink grant to be sent by a target cell;detect the uplink grant in the identified subframe transmitted by atarget wireless access network node of the target cell; and send, inresponse to the UE successfully accessing the target cell or detectingthe information relating to the uplink grant, an RRC message to thetarget cell to handover from the source cell to the target cell.
 20. Thenon-transitory machine-readable storage medium of claim 19, wherein theRRC message received from the source wireless access network node is anRRC Reconfiguration message, and the RRC message sent to the target cellis an RRC Reestablishment message.